Race condition with local variable in @threads for loop

General Usage
1

Hi, im trying to figure why this code throw and AssertionError inside the most inner loop. My current understanding is that julia will split the top loop into a number of pieces and spawn each piece to a separate thread. So, I just interpret the whole code inside the top loop as always running in the context of a given task.

Why then is the second assertion triggered?
Does julia spawn the inner for loop too?
If so, How can I avoid it?

# test.jl
import Base.Threads: @threads, nthreads

function test()
    @threads :static for i in 1:100

        d = Dict(:a => Dict())
        a = d[:a]
        @assert a === d[:a]

        # Line 12: This throw an AssertionError
        for j in 1:100; @assert a === d[:a] end 
    end
    a = [] # If this line is commented the problem disappears 
end

println("Running test (-t$(nthreads()))")
for i = 1:500; test() end

$ julia -t4 test.jl
ERROR: LoadError: TaskFailedException:
AssertionError: a === d[:a]
Stacktrace:
 [1] macro expansion at /Users/Pereiro/Documents/dev/threads_issue/test.jl:12 [inlined]
 [2] (::var"#41#threadsfor_fun#1"{UnitRange{Int64}})(::Bool) at ./threadingconstructs.jl:81
 [3] (::var"#41#threadsfor_fun#1"{UnitRange{Int64}})() at ./threadingconstructs.jl:48
[...]

Something strange that makes me think I’m missing something is that if I comment the a=[] line, the error doesn’t happen.

My version info

$ julia -e "using InteractiveUtils; versioninfo()"
Julia Version 1.5.2
Commit 539f3ce943 (2020-09-23 23:17 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin18.7.0)
  CPU: Intel(R) Core(TM) i5-8210Y CPU @ 1.60GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-9.0.1 (ORCJIT, skylake)
Environment:
  JULIA_NUM_THREADS = 4

Any help will be appreciated :slight_smile:

Thanks!

2

https://github.com/JuliaLang/julia/issues/14948

The last assignment makes the variable a have a scope outside the threads section.

3 Likes
3

a has to be strictly local to @threads for. Your code seems to make it an existing local variable (see Kristoffer’s comment). Then threads interfere with each other. Make it explicitly local and it runs.

function test()
    @threads for i in 1:100
        ...
        local a = d[:a]
        ...
    end
    a = [] # ...
end

PS: edited

1 Like
4

Uff, I wouldn’t figured out by myself, thanks! I will change the title for improve findability, any suggestion?

1 Like
5

ok, I have clarified my response.

Perhaps you could call it “Race condition with local variable in @threads for”

1 Like
6

FYI @floop ex for from FLoops.jl can be used instead of @threads for and it’d detect this error. It also shows which variable is causing it:

image
image1155Ă—426 44.9 KB

@floop ex for workarounds common pitfalls of @threads for like this one, provides a rich reduction API, and supports multiple backends including GPU, with FoldsCUDA.jl.

7 Likes